The distribution of television signals has increasingly become based on digital methods and digitally encoded forms of video and audio signals. At the same time, higher resolution (high definition TV) has become available in the market place, commensurate with larger and higher definition displays. To meet the requirement of interconnecting such high definition displays with digital signal sources such as Digital Versatile Disc (DVD) players and receivers/decoders for digital satellite and digital cable distribution of video material, a digital interface standard has evolved, known as the High-Definition Multimedia Interface (HDMI). A detailed specification for HDMI can be obtained from the “hdmi.org” website. The HDMI specification currently available and used in this application is HDMI specification version 1.3 dated Jun. 22, 2006, which is incorporated herein by reference. This HDMI standard can be employed for connecting digital video sources to digital video sinks over a cable that carries a number of digital signals and a clock signal.
The inherent characteristics and manufacturing imperfections of high-speed differential signaling cables such as may be used to carry HDMI signals have an adverse effect on the high-speed signals carried by the cable.
For example, any cable has a limited bandwidth and therefore acts as a low pass filter. The bandwidth of the cable is related to its length, the longer the cable the greater the filtering effect and the lower its bandwidth. As a result, high-frequency signals passing through the cable are attenuated, and their edges become less sharp. This leads to an increased risk of misinterpreting the received data at the receiver end of the cable, especially for long cables and high-speed data.
FIGS. 1A-1C illustrate the effect of the limited bandwidth of a cable on the transmitted signals. FIG. 1A illustrates a high-speed signal to be transmitted through a high-speed cable, FIG. 1B shows a distorted bandwidth-limited signal received at the receiver end of the cable (before equalization), and FIG. 1C shows the received signal at the receiver end after equalization. As seen from FIG. 1B, the signal edges are slowed and short pulses are narrowed, not reaching the full transmitted amplitude.
Differential signaling cables are commonly used to carry high-speed digital signals in differential form, that is pulses of opposing polarities are transmitted on the two strands of the cable. The differential signal carried over such cables may be warped, that is the two signal components (positive and negative polarities V+ and V−) are skewed in time with respect to each other (differential skew), further distorting the received signal.
The impact of differential skew is depicted in timing diagrams in FIGS. 2A and 2B.
FIG. 2A shows an example timing diagram of the two single ended signal components (V+, V−) of the differential data on an HDMI channel, as it may be transmitted by an HDMI source into a cable. A timing diagram of the corresponding differential signal (Vdiff-xmit) in FIG. 2A illustrates the corresponding differential signal that is clean and easily interpreted.
FIG. 2B shows an example timing diagram of the two single ended signal components (V+ and V−del) of the differential data on an HDMI channel, as it might be received at the end of a cable. For the sake of clarity, only the effect of the differential skew is shown in FIG. 2B. The signals V+ and V− are skewed in time with respect to each other. The negative signal component V− is delayed with respect to the signal component V+ by a differential skew delay of Td. A timing diagram of the corresponding distorted differential signal (Vdiff−rcv) in FIG. 2B illustrates that, as a consequence of the differential skew, the differential signal Vdiff−rcv is significantly distorted with clearly visible plateaus in the signal where the differential signal is zero (0). These plateau regions can only be interpreted as noise by the receiver, the result of which is to reduce the width of the window of valid data. This reduction is seen as a closure of the receive data eye and directly compromises the channel quality. The amount of differential skew delay (Td) depends on the characteristics of each individual cable, and is basically constant.
Earlier approaches to improving cable quality so far have been limited to embedded passive equalizer circuits within the cable, which boost high frequencies of the signals attenuated in the cable. Such equalizers are fixed to compensate for a fixed cable length.
While the equalization required for a given cable depends largely on the length of the cable, other characteristics of high-speed signaling cables such as the differential skew, being more random, may vary substantially between the cables.
Accordingly, there is a need in the industry for the development of an improved high-speed signaling cable, which would provide improved signal characteristics.
Earlier High-Definition Multimedia Interface (HDMI) signal boosters that can be used to boost HDMI signals use external power inputs, see e.g. Long Reach™ product of Gennum corporation, which has been submitted in an Information Disclosure Statement. As a result, they cannot be embedded in a standard HDMI cable. A more recent development is a stand-alone “super booster” that can be inserted inline with a cable, and is also available integrated in an HDMI cable, see references: Gefen Inc., submitted in an Information Disclosure Statement, including an advertisement of a standalone HDMI “super booster; A manual for the standalone HDMI “super booster, which has been submitted in an Information Disclosure Statement; and an advertisement for a cable with an integrated HDMI “super booster” submitted in an Information Disclosure Statement.
The possibility of embedding an active device within the cable is associated with a problem. Firstly, no power input may be available for such a device except through the cable, i.e. there is no provision for external power supplies. Secondly, in the case of the HDMI cable, there is not enough power available to power a simple signal regenerator, primarily because of the specification requirement to provide a termination voltage for the inputs. As a result, the embedded active device apparently cannot be powered as required.
In more detail, the main power requirement for an HDMI signal booster is the requirement to provide a termination voltage (3.3V) with the capability to source 12 mA for each of three HDMI inputs. The power that is available from the cable comes from a 5V line, from which a maximum current of 5 mA can be drawn (as per HDMI specification V1.3) when the sink device is active, i.e. the total available power is limited to 50 mW. The combined power requirement of the input terminations on the other hand is approximately 12 mA*3.3V*3=120 mW. Unfortunately, these requirements cannot be met in a standard HDMI cable in a simple way.
Accordingly, there is a need in the industry for the development of an improved signal booster with an improved power control circuit for embedded cable applications based on one or more active devices, which would avoid or mitigate the above noted problem.